The present invention relates to a method to guarantee for a service a bandwidth across an access network with a quality of service, and to an access network wherein the method is applied.
Such a method is already known in the art, e.g. from the document entitled ‘Traffic Management Specification’, published by the ATM forum technical committee on March 1999, version 4.1, reference AF-TM-0121.000. Therein, a method to achieve performance objectives in an Asynchronous Transfer Mode (ATM) network and to protect the network from traffic congestion is described.
An access network is that part of a data communications network that connects the subscribers to the core network. The access network may overlay upon an established network such as the Public Switched Telephone Network (PSTN), typically upon the last few miles of the PSTN to serve thinly scattered residential subscribers. The trade-off is to get the highest traffic concentration ratios across the access network, i.e. less equipment and less leased lines, while guaranteeing the best network performances to the subscribers.
ATM is a candidate technology for the transport layer between the subscriber and the core network. ATM is a connection-oriented cell-based transport technique and is intended to support a wide variety of services and applications. At the subscriber premises, a Data Communication Equipment (DCE) chops the upstream traffic from a Data Terminal Equipment (DTE) into data blocks, encapsulates the data blocks into ATM cells and transmits the ATM cells over the User Network Interface (UNI). The ATM cells are transported over a virtual connection, which is set up across the access network between the DCE and an edge node of the access network coupled to the core network. The edge node extracts the upstream traffic from the ATM cells and transmits it over the core network. In the reverse direction, the edge node encapsulates the downstream traffic from the core network into ATM cells and transmits them over the appropriate virtual connection. The edge node could also stand for:                a network element of the access network that is coupled to the PSTN, such as a voice gateway,        a network element of the access network that houses a service provider or that is coupled via a Local Area Network (LAN) to a service provider,or any alternative as known to the skilled person.        
ATM defines a two-level hierarchical connection scheme:                the Virtual Channel (VC) is a connection between two end systems for a specific service or application,        the Virtual Path (VP) is a connection between two network nodes that transports a large number of simultaneous VCs.        
A connection can be initiated by any of the two end systems involved in the connection or can be provisioned by a third party, such as a network manager. In the latter case, the connection is referred to as Permanent Virtual Channel (PVC) or Permanent Virtual Path (PVP).
The connection-oriented mode of operation allows the network to check whether enough resources are available for a connection and then to commit some Quality of Service (QoS) for that connection. The bandwidth and the QoS required for a connection are negotiated at connection establishment time. The network can reserve therefrom the appropriate resources for this connection. The network guarantees that the congestion of the network resources will only occur with a very low pre-determined probability.
An object of the known method is to meet bandwidth and QoS requirements across an ATM network. Another object is to promote the efficient use of the network resources.
In a preliminary step of the known method, a set of parameters is defined for describing the bandwidth and the QoS requirements.
The bandwidth requirements are expressed by means of traffic descriptors featuring the way a source may transmit cells over the network. The Peak Cell Rate (PCR) defines the maximum cell rate a source may reach. In case of fluctuating cell rate (e.g., because of a video compression algorithm), additional descriptors might be specified to allow the network to allocate resources more efficiently.
The quality of service of a connection relates to the cell loss, the delay and the delay variation (also called jitter) incurred by the cells transferred over the connection.
Thereupon, appropriately differentiated service categories are defined. Whenever a connection is set up, the service category to which that connection relates and the traffic and QoS parameters relevant to that service category are specified.
The ATM service categories are:                Unspecified Bit Rate (UBR) service category,        Constant Bit Rate (CBR) service category,        real time—Variable Bit Rate (rt-VBR) service category,        non-real time—Variable Bit Rate (nrt-VBR) service category,        Available Bit Rate (ABR) service category,        Guaranteed Frame Rate (GFR) service category.        
The UBR service category is intended for non-real time best effort applications, such as internet browsing, file transfer or e-mail.
The CBR service category is used by connections that request a static amount of bandwidth that is continuously available during the connection lifetime. This amount of bandwidth is characterized by the PCR value. The source may emit cells at or below the negotiated PCR (and may also even be silent) at any time and for any duration and the QoS commitments still pertain. CBR service is intended for real time applications requiring tightly constrained delay, delay variation and cell loss ratio, such as video streaming or Voice over IP (VoIP).
Refer to the above mentioned specification for further details about the remaining service categories and the set of parameters relevant to each category.
In a further step of the known method, mechanisms are specified for managing and controlling traffic and congestion in ATM networks. These mechanisms are:                Connection Admission Control (CAC), which is defined as the set of actions taken by the network during the connection set up phase to determine whether a connection request can be progressed or should be rejected according to the network resources that remain available for that connection,        Usage Parameter Control (UPC), which is defined as the set of actions taken by the network to monitor and, possibly, to enforce the traffic contract at the UNI,        Network Parameter Control (NPC), which is defined as the set of actions taken by the network to monitor and, possibly, to enforce the traffic contract at the Network Node Interface (NNI),        Feedback control, which is defined as the set of actions taken by the network and by the end-systems to adjust the traffic submitted on a ATM connection according to the state of the network elements along that connection,        Cell Loss Priority (CLP) control, which allows the network to discard some lower priority cells to protect, as far as possible, the QoS objectives of higher priority cells,        Traffic Shaping, used to achieve a desired modification to the traffic characteristics of a connection.        
The main objective of the UPC and NPC policers is to protect the network resources from malicious, as well as unintentional misbehaviour, which can affect the QoS of other established connections. This protection is achieved by detecting violations of the negotiated traffic parameters and by taking appropriate actions, such as discarding cells.
Most of the ATM based access networks make use of PVCs to connect a subscriber's DCE to an edge node of the access network. As a matter of fact, the installed base of subscriber's DCEs are for the most part unable to set up an ATM connection on their own. They miss important ATM features (no signalling channel, etc) as well as the necessary pieces of information (no ATM addressing data, etc).
A subscriber is provisioned with as many PVCs as there are classes of service to support for that subscriber. Each PVC is assigned a suitable service category in accordance with the known method.
A shortcoming of the know method is the very loose integration between those PVCs overlaying upon the access network and adapted to transport an amount of traffic across the access network with a QoS, and the higher communication layers submitting traffic thereto. The traffic submitted over a virtual connection does not necessarily need to conform to the ATM traffic contract of that virtual connection. If the real traffic load goes beyond the provisioned bandwidth, a UPC or NPC policer will start discarding ATM cells irrespective of the service to which the ATM cells relate. Consequently, delivering a new service over a connection without any further check may alter the integrity of existing services delivered over that connection.